uri_parser: Fix relative recognition
uri_parser: Fix relative recognition

Relative refernces with colons in their names can not be told by their
first character alone.

jasonatran/RIOT,OTAkeys/RIOT,authmillenon/RIOT,ant9000/RIOT,jasonatran/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,kaspar030/RIOT,OlegHahm/RIOT,yogo1212/RIOT,ant9000/RIOT,basilfx/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,miri64/RIOT,ant9000/RIOT,basilfx/RIOT,miri64/RIOT,ant9000/RIOT,basilfx/RIOT,jasonatran/RIOT,OTAkeys/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,basilfx/RIOT,kYc0o/RIOT,yogo1212/RIOT,yogo1212/RIOT,authmillenon/RIOT,basilfx/RIOT,kaspar030/RIOT,OTAkeys/RIOT,miri64/RIOT,OTAkeys/RIOT,kYc0o/RIOT,jasonatran/RIOT,authmillenon/RIOT,ant9000/RIOT,OlegHahm/RIOT,kYc0o/RIOT,kaspar030/RIOT,miri64/RIOT,miri64/RIOT,OTAkeys/RIOT,yogo1212/RIOT,authmillenon/RIOT,authmillenon/RIOT,kYc0o/RIOT,kaspar030/RIOT,yogo1212/RIOT,yogo1212/RIOT,jasonatran/RIOT,authmillenon/RIOT